Detection devices for identifying chemical species and other materials are known in the art. Spectroscopy has been utilized in detection and identification applications. Conventional material or chemical detection technologies can be classified into general categories.
A first category includes lidar techniques which reflect or scatter a laser beam from a scene. The laser beam is tuned across a spectroscopic absorption feature of the target gas and differential absorption of returning photons is used for target detection. Because of the complex laser technology utilized to provide appropriate power (e.g., optical parametric generators), these systems are typically built to sense only one or a few compounds. The use of a laser beam precludes use of these techniques in covert applications. A recent development in this technology is the availability of simple laser diodes, however these diodes are minimally tunable and therefore are essentially built to detect single compounds.
Another category comprises open-path FTIR (Fourier transform infrared) spectrometers. Such spectrometers often use a broadband light source in combination with a Fourier transform spectrometer. The light sources employed are typically weak and require a high reflectance mirror. These systems have not yet achieved a desirable level of robustness or sensitivity achievable with lidar systems.
Other spectrometry techniques utilize passive infrared. These infrared systems typically use a spectrometer and highly sensitive detector to examine passively occurring emission or absorption lines. This technique has the benefits of being inherently covert and capable of sensitivities comparable to active techniques.
Techniques employing conventional spectrometry typically use dispersive, refractive, or interference-filter based optics. Such spectrometers have the disadvantage of requiring careful calibration to certain wavelength regions in order to admit only the spectral line of interest.
Another detection method includes reference cell spectroscopy or gas filter correlation (GFC) spectroscopy. Conventional reference cell spectroscopy utilizes a reference gas cell on a rotating platform which can be configured to alternately block and unblock incoming light to produce modulation in the input and output of a downstream detector. Alternatively, an alternating/rotating beamsplitter can be utilized.
The basic concept is that the degree of modulation due to insertion of a reference cell depends upon whether or not the input spectrum has spectral features that correlate with those of the absorption cell. This conventional technique is severely limited by a host of instrumental problems including those resulting from the use of multiple optical paths.
In this technique, incoming light containing spectral information is alternatingly transmitted over separate optical paths. One path contains a cell having a gas to be analyzed and the other path serves as a control. A light switch or mechanical method is utilized to change from one path to the other. The light intensity of the two paths is compared.
Problems exist with the utilization of this conventional technique. The technique is subject to leg imbalance due to various drift effects. A lack of balance between the signal and reference optical paths makes it difficult to determine whether a detected "signal" is actually due to the spectral lines of interest or the lack of proper calibration or balance of the instrument. In addition, the intensity balance of two-leg systems which can be maintained over a substantial period of time is about 0.5 percent of the optical intensity in either leg. Thus, detected absorption over substantial time periods is limited to approximately this level.
Therefore, there exists a need to provide improved apparatuses and methods for detecting materials, such as, for example, chemical compounds, which avoid the problems associated with the prior art.